CoC? I found this on Wiki. I'm not sure if this explains it.
Only the President can direct the use of nuclear weapons by U.S. armed forces, including the Single Integrated Operational Plan (SIOP). While the President does have unilateral authority as commander-in-chief to order that nuclear weapons be used for any reason at any time, the actual procedures and technical systems in place for authorizing the execution of a launch order requires a secondary confirmation under a two-man rule, as the President's order is subject to secondary confirmation by the Secretary of Defense. If the Secretary of Defense does not concur, then the President may in his sole discretion fire the Secretary. The Secretary of Defense has legal authority to approve the order, but cannot veto it.